1. It appears that pepsin releases kinin only from kininogen molecules which contain the kinin moiety at the carboxyl end. 2. A radioimmunoassay for human urinary kallikrein is under development. Values obtained with this assay do not agree well with values for total kallikrein determined by the radiochemical esterase assay, yet the esterase can be removed from urine by immunoadsorption using an adsorbent containing monospecific antibodies to human urinary kallikrein. 3. A radioimmunoassay for urokininogen is under development. Values obtained with this assay do not agree well with values obtained for urokininogen based on bioassay of kinin release by trypsin or hog pancreatic kallikrein. 4. A radioimmunoassay for total plasma kininogen is under development. Values obtained with this assay do not agree well with values obtained by bioassay of kinin released with trypsin. High molecular weight kininogen in plasma can be accurately and reliably determined by radioimmunoassay (ZO1 HL 01022-02 CH). High dietary salt appears to cause an elevation and administration of Fludrocortisone does not appear to change the plasma levels. One of four patients with hereditary angioedema had only half the normal level of plasma high molecular weight kininogen.